Flake metal powders coated with fluorocarbon resin



United States Patent 3,389,165 FLAKE METAL POWDERS COATED WITHFLUOROCARBON RESIN William T. llolger, Chathatn, N.J., assignor to AlcanMetal Powders, Inc., a corporation of Delaware N0 Drawing. Filed Mar. 5,1965, Ser. No. 437,569 37 Claims. (Cl. 260Z3) ABSTRACT OF THE DISLOSUREProcedure for making flake metal powder, wherein finely divided metal isground to flake form in a ball mill, stamping mill or the like, in thepresence of a fluoro-- carbon resin as a grinding agent. Thefluorocarbon resin may be polytetrafluoroethylene, fluorinated ethylenepropylene, polyvinylidene fluoride or polychlorotrifluoroethylene, andmay be used either alone, as the sole grinding This invention relates toflake-type metal powders and to methods of making such powders.

Metal powders in the form of minute flakes are widely used as metallicpigments in inks, paints and the like, i.e. in dispersion in a suitablefilm-forming vehicle to provide a protective or decorativesurface-coatlng composition. Examples of such flake metal powders arethe socalled gold bronze powders (the term gold bronze being employed todesignate flake metal powders either of copper or of brass containinge.g. up to about zinc) and aluminum pigments.

These powders are commonly made by grinding finely divided metal, suchas foil scrap or atomized powder, in a ball mill, stamping mill or thelike which hammers the metal particles into the desired flake form. Thisgrinding may be done either dry, in air or other gas, or wet in someliquid such as mineral spirits. For the preparation of gold bronzepowders, dry milling is ordinarily preferred; in such operation, thefinely divided metal can be blown into the mill and the produced flakepowder blown out in dry form, facilitating handling and sizeclassiiication of the product. On the other hand, wet milling ispreferred for the preparation of flake aluminum powder, owing to thefact that aluminum powder (unlike gold bronze) may explode during drygrinding unless extreme caution is used.

In both wet and dry grinding operations, it is necessary to perform thegrinding or milling step in the presence of a small amount of a grindingagent. Various oils, fats and the like, such as olive oil, tallow andlard oil, have been proposed for this purpose; in present-day commercialpractice, fatty acids (for example stearic acid and oleic acid) are verywidely employed as grinding agents, although a number of other materialsare also used, e.g. zinc stearate and other derivatives and mixtures offatty acids. The grinding agent acts to protect the metal particlesduring grinding, so that they are flattened into the desired flake formrather than merely being broken up, and to prevent cold welding of theparticles. In addition, the grinding agent coats the particles with athin film; this coating (which remains on the particles after grindingand is presently believed to be chemically attached to the flakesurfaces, at least in part) serves to protect the flakes from corrosionor other deleterious chemical attack "ice and thereby aids in preservingthe brightness or luster of the flake pigment.

The grinding agent may also have the effect of giving the flakesso-called leafing properties. In this connection it may be explainedthat leafing is a property whereby in a paint or ink vehicle or thelike, the flakes float to the surface of the vehicle and tend to lie infiat, parallel or overlapping relation at the surface, thus forming amore reflective and impervious coating than nonleafing flakes which aredisposed in random attitudes when dispersed in a vehicle. Accordingly,leafing powders are employed when a highly metallic appearance isdesired for a paint or ink, while ncn-leafing powders are used when itis desired to impart metallic luster without bare .tetailic appearanceto a surface coating. Stearic acid is conventionally employed as agrinding agent to produce leafing flakes, particularly in the productionof gold bronze, where leafing properties are usually desired; oleic acidis commonly used as a grinding agent to produce a nonleaflng flakeproduct e.g. in the manufacture of aluminum flake pigments for use inautomobile finishes and the like. Leafing properties, it may be furtherexplained, are believed to be attributable to the surface coatingimparted to the flakes by the grinding agent and in particular tosurface tension effects produced by the: coating.

A diificulty heretofore encountered in the production of flake metalpowders, and especially in the production of gold bronze using agrinding agent such as stearic acid, is that the product tends to beoverground, i.e. to contain an excessive amount of super-fines; thesefines tend to darken the product and give it a dirty appearance. That isto say, agents such as stearic acid appear to have a preferential effecton fine particles in the feed of divided metal, so that there is astrong tendency to produce in the mill a mixture of coarse and fineflakes which is difiicult to separate into desired grades of relativelyuniformly sized flakes; by the time most of the particles aresufficiently thinned out, those that were first struck and flaked havebeen over-ground or broken down to a size too small to reflect light,and therefore appear dark, dulling the luster of the produced flakepigment. This tendency has heretofore been counteracted by using extremecare in the addition of the grinding agent and by attempting tocontinuously classify and remove the finer flakes as they are produced.Such methods, however, are necessarily expensive and only partlysuccessful.

While the reason for this production of superfines is not fullyunderstood, it is presently believed that conventional griding agentssuch as stearic acid, after adsorbing on the surface of the dividedmetal in the mill, may tend to migrate into micro-cracks or to affectcrystal defects and thus have a disruptive effect close to the surfaceof the metal particles. Such near-surface effect would, of course, havegreater effect on smaller particles.

An object of the present invention is to provide new and improvedprocedures for making flake metal powders. Another object is to providesuch procedures wherein production of excessive lines is avoided, and aproduct of advantageously superior brilliance thereby achieved, in afacile and convenient manner not requiring close or critical control ofoperating conditions. A further object is to provide such procedures,which can be used to produce either leafing or non-leafing flakes, andsuitable for either Wet or dry milling operation. Yet another object isto provide procedures for making gold bronze pigments having improvedproperties especially with respect to brightness and freedom from fines.A still further object is to provide procedures for making flakealuminum pigments of improved characteristics. An additional object isto provide new and improved flake metal pigments.

To these and other ends, the present invention in a broad sensecontemplates the use of fluorocarbon resins as grinding agents in themanufacture of flake metal powders. The term fluorocarbon resins (seeModern Plastics, Encyclopedia issue for 1965, vol. 42 No. 1A, pp.119-120) as used herein refers to fluorinated, so-called linearpolyolefins, including polytetrafluoroethylene, fluorinated ethylenepropylene (tetrafluoroethylene-hexafluoropropylene copolymer),polyvinylidene fluoride, and polychlorotrifluoroethylene. It is foundthat these resins constitute very effective grinding agents for suchpurpose; i.e. when 'used in place of or in addition to a conventionalgrinding agent such as stearic acid or oleic acid in grinding finelydivided metal as in a ball mill or stamping mill, these resins act toprotect the particles so that they are reduced to the desired flakeform, and are also understood, from evidence of effects produced, toprovide a surface coating on the flakes which coating is thus understoodto afford the effective or indeed superior protection against corrosionthat has been observed with the product. References hereinbelow to thesurface coating of the product of the present invention will beunderstood in this context.

Moreover, leafing flake powders can be made with use of thesefluorocarbon resins as grinding agents. In the case of gold bronze, thefluorocarbon resin grinding agents are themselves capable of impartinggood leafing properties to the produced flakes, i.e. when used alone; inother instances, as in the production of leafing aluminum pigments, thefluorocarbon resin grinding agents may be used in conjunction with aleaf-producing grinding agent such as stearic acid without interferingwith the function of such leaf-producing agent. At the same time, anon-leafing aluminum pigment (i.e. a pigment having no leaf, asdemonstrated by inability to be supported on the surface of xylene atroom temperature, this being a desirable property for aluminum pigmentsfor automobile finishes and the like), having superior corrosionresistance, can be produced by wet-milling aluminum with a fluorocarbonresin used alone as a grinding agent.

It is further particularly found that the use of fluorocarbon resins asgrinding agents affords very significant advantages with respect tofreedom of the flake product from fines. That is to say, by grinding inthe presence of a fluorocarbon resin, an advantageously brilliant flakeproduct (free of excessive fines which would darken or dirty theproduct) is very readily achieved, without close or critical control ofoperating conditions (e.g. time of milling) as has heretofore beennecessary to minimize the occurrence of fines in flake metal pigmentssuch as gold bronze produced e.g. with stearic acid. This flake productobtained with the present invention is more uniform in paiticle sizethan that produced by previously proposed methods, as with stearic acid,and can more readily be separated into desired grades of relativelyuniformly sized flakes. Moreover, especially in the case of gold bronze,this product exhibits improved flowing properties, as when dispersed inan ink vehicle for printing or other such application.

Broadly, then, the method of the invention comprises grinding finelydivided metal to flake form in the presence of a fluorocarbon resin orresins as a grinding agent. The grinding procedure may be performed in aball mill, stamping mill or the like, i.e. in equipment as presentlyused for grinding flake metal powders, operated in conventional mannerand with conventional steps of supplying feed of divided metal andgrinding agent to the mill and withdrawing flake product therefrom. Inother words, except for the use of the fluorocarbon resin as part or allof the grinding agent, the grinding operation and desired preliminaryand subsequent treatments may be carried out in essentially the samemanner, and with the same techniques and equipment, as in methodsheretofore employed for making metal flakes using conventional grindingagents, such operation and equipment being well known to those skilledin the art.

In the method of the invention in this broad sense,

the feed of finely-divided metal may be any metal which it is desired toreduce to flake form. The fluorocarbon resin or resins employed may beused either alone or in mixture with other grinding agent or agents,e.g. stearic acid or oleic acid, it being understood that the termgrinding agent as used herein broadly refers to any material having theproperty of protecting finely-divided metal during milling so that theparticles are reduced to flake form, and suitable for use in grindingflake metal powders. More particularly, in the preferred practice of theinvention the grinding agent used is one of the following fluorocarbonresins: polytetrafluoroethylene, fluorinated ethylene propylene(tetrafluoroethylene-hexafluoropropylene copolymer), polyvinylidenefluoride, and polychlorotrifluoroethylene. Whereas all these resins havebeen found to be effective as grinding agents, materials found to affordespecially good results are the perfluorinated linear polyolefins, i.e.including polytetrafluoroethylene and fluorinated ethylene propylene; ofthese, it is presently particularly preferred to usepolytetrafluoroethylene, the latter resin being readily commerciallyavailable at relatively low cost and, further, being highly effective inproviding the advantages of the invention. Very preferably, also, thefluorocarbon resin used is supplied to the mill in finely divided formin a minor proportion based on the feed of divided metal to the mill.Thus, for example, the fluorocarbon resin may conveniently be used indry powder form; alternatively, the fluorocarbon resin may be suppliedto the mill in finely-divided form in a suspension, as with an agent orcarrier appropriate for the selected type of milling.

The method of the invention embraces both dry-milling and wet-millingoperation; i.e. the feed of metal particles /ith the fluorocarbon resinmay be ground either dry, or wet in the presence of a liquid wet-millingvehicle (commonly an organic liquid) such as mineral spirits. Suitableliquid wet-milling vehicles are those convention ally employed forwet-milling flake metal powders, such vehicles being well-known in theart. The proportion of grinding agent used, while not critical, may varyin accordance with the milling conditions employed. Thus in dry-millingor flake metal powder, a convenient range of proportions for thesupplied fluorocarbon grinding agent (based on the feed of dividedmetal) is between about and about 4% by weight, a presently preferredrange being from about /2 to about 2%; in wet milling, a somewhat largerproportion of fluorocarbon resin may be used, e.g. about 1% up to about10% by weight (preferably about 2% to about 6%), since much of thegrinding agent remains in suspension in the liquid used in the wetmilling.

The product of the described method is a flake metal powder having asurface coating of the fluorocarbon resin used as grinding agent. Thisproduct may be either leafing or nonleafing in character, and may besubjected to further finishing operations if desired; for example, itmay be polished, in accordance with procedures heretofore known, toimprove its brilliance and leafing properties.

While, as stated, in a broad sense the method of the invention isapplicable generally to the preparation of flake metal powders, it isfound to be especially advanta geous for the manufacture of gold bronzepowders, and accordingly the latter application constitutes an importantspecific aspect of the invention. In this aspect, the inventioncontemplates grinding to flake form a finely divided gold bronze feed(copper, or brass-i.e. copper-zinc alloy-containing e.g. up to about 30%zinc), in the presence of a fluorocarbon resin grinding agent, asdescribed above, in finely divided form. As indicated above, thisgrinding operation may be performed with conventional equipment, such asa ball mill or stamping mill, preferably under dry-milling conditions tofacilitate handling and size classification of the product.

Thus, in an exemplary instance of such operation, incorporatingpresently preferred features, a feed of metal powder of an appropriatealloy to make gold bronze pigment, of for example minus 100 mesh (Tylerscale) particle size, is continuously blown into a ball mill, togetherwith polytetrafluoroethylene powder in a proportion conveniently rangingbetween about /a% and about 2% by weight (eg, a proportion of about 1%)based on the weight of metal to be flaked. As the mill is continuouslyrotated to grind the feed of metal to flake form, flaked powder iscontinuously blown out of the mill and classified as to size in anyappropriate manner, e.g. with a conventional screen or air classifier.The desired size grade of flakes (e.g. flakes of -325 mesh particlesize) is removed as the product, and the larger flakes are returnedtothe mill for further grinding.

The gold bronze flake pigment thereby produced is found to be ofsuperior brilliance, i.e. containing an advantageously lower proportionof fines than gold bronze made for example with stearic acid as agrinding agent; of more uniform size than heretofore obtainable; andmore readily classifiable into uniform size grades. In addition, withthe present method a -gold-bronze product can be achieved having asignificantly higher apparent density than the stearic acid product.That is to say, whereas gold bronze ground with stearic acid in a ballmill may have an apparent density of about 12 to 14 g./in. or up toabout 16 g./in. or above when ground in a stamping mill, the product ofthe present method may have an apparent density ringing up to about 24g./in. Because of this greater apparent density, the gold bronze flakeproduced by the present method exhibits desirably superior flowingproperties when dispersed in an ink or like vehicle; that is to say,this pigment is more free-flowing and hence more readily transferred (inink) from printing rolls to paper than the gold bronze products madewith stearic acid. It is further found that with the described operationthere can be produced a flake having good leafing properties.

These desirable properties are achieved by the present method withoutthe necessity for such close or critical control of operating conditionsas has heretofore been necessary in making gold bronze, e.g. withstearic acid to avoid excessive fines in the product. Further, theinvention provides these advantages even when the feed is socalled richbronze feed (containing above about 25% zinc) which has heretofore beenparticularly difficult to grind to flake form without production ofexcessive fines.

The product of the above-described example of operation is a gold-bronzeflake having thereon a surface coating of polytetrafluoroethylene,effective to provide leafing in typical surface-coating vehicles.

To enhance the leafing properties as well as the brilliance of theflakes, the gold bronze product of the present method is preferablypolished after grinding, as in accordance with present conventionalpractice for treatment of gold bronze pigments ground by methodsheretofore known. Such polishing may be performed in a so-called brushpolisher or other device wherein the flakes are gently rubbed so thatwhen made into a coating wi:h an ink or paint vehicle they form asmoother and more brilliant sur-face. During this polishing operation,stearic acid or other polishing agents such as are now used may beadded. Use of such conventional polishing agent (e.g. stearic acid) inthe polishing step is found to improve the properties of the product,and in particular to increase the water coverage of the flake product(i.e. the area of water covered with a layer one flake thick, per unitweight of flake product). However, where a film of polishing agent isnot desired on the product flake, the latter agent may be omitted in thepolishing step.

Although the foregoing procedure as applied to the manufacture of goldbronze pigments has been described above with reference to the use ofpolytetrafluoroethylene alone as a grinding agent, it will beappreciated that other fluorocarbon resins, eg those named above, may beused in place of polytetrafluoroethylene, and similarly, that the latteror other fluorocarbon resin may be used in mixture with a conventionalgrinding agent such as stearic acid, in each case with the advantagesset forth above. Similarly, while ball-milling operation has beendescribed, the grinding operation may be performed in a stamping mill orother grinding equipment suitable for reducing finely divided metal toflake form.

In a further specific aspect, the invention is concerned with theproduction of flake aluminum pigment, and embraces the grinding offinely divided aluminum (e.g. foil scrap) to flake form in the presenceof a fluorocarbon resin as described above, as grinding agent,preferably under wet-milling conditions (viz. in a liquid wet-millingvehicle such as mineral spirits or hydrocarbon solvent). It may beexplained in this connection that in the con ventional grinding of flakealuminum pigments, as with stearic acid, problems are encountered due tothe accumulation of dark metal superfines and a contamination (probablyaluminum stearate) which together make the product darker and lessleafing; to meet these problems it has been necessary either to performan expensive distillation or other purifying treatment of the mineralspirits, or to completely discard and replace the mineral spirits atfrequent intervals. Use of a fluorocarbon resin as grinding agent inaccordance with the present invention is expected to eliminate or atleast very materially reduce these problems, since comparatively littlesuperfine material is produced to react with the solvent and grindingagent, and the fluorocarbon grinding agent is far less reactive than thefatty acids heretofore used.

In this regard it may be noted that grinding agents such as stearic acidheretofore used in wet-milling aluminum flake pigments dissolve in thehydrocarbon solvent used as the wet-milling vehicle; ie the vehicle actsas a solvent for the stearic acid, although in some cases not all thestearic acid may dissolve. The present fluorocarbon resin grindingagents (e.g., polytetrafluoroethylene), however, do not dissolve in thehydrocarbon solvent wet-milling vehicle, but instead remain insuspension therein, and hence the vehicle does not act as a solvent butmerely serves as a carrier for the fluorocarbon resin.

The grinding of aluminum flakes by the method of the present inventionmay be carried out in conventional equipment and in accordance withconventional procedures for wet-milling aluminum flakes, except that afluorocarbon resin (for example polytetrafluoroethylene) is used inplace of or in addition to the grinding agents heretofore employed. Theproduct thereby obtained is a flake aluminum pigment having a coating ofthe fluorocarbon resin used, e.g. a coating of polytetrafluoroethylene.This product is found to have superior resistance to corrosion, such asattack by hydrochloric acid, apparently due to the protective effect ofthe present coating. Either a leafing or non-leafing product may beobtained by varying process conditions; for example, a. leafing productcan be made by use of supplemental grinding agents such as stearic acidin conjunction with a fluorocarbon resin or a non-leafing product can bemade by omitting such agents and using the fluorocarbon resin alone asgrinding agent.

Other examples of metals made in flake form and to which the presentinvention is applicable include gold, iron, stainless steel, nickel,tin, chromium, lead, bismuth, and various alloys of these metals.

Further features and advantages of the invention will be apparent fromthe following specific examples of production of flake metal powders inaccordance with the present method. In these examples, percentage values(where given) of grinding agents used in milling metal powder feedsrefer to the weight of grinding agent supplied to the mill, expressed asa percent of the weight of the powdered metal feed.

EXAMPLE I In a series of runs, gold bronze feed was dry-milled to flakeform with polytetrafluoroethylene (TFE) powder in a steel jar 6 inchesin diameter and 3 inches deep, having 3 lifter bars Mi inch square, andabout half-filled with 2 to 3 kg. of polished steel balls A: to inch indiameter, by rotating the jar (about a horizontal axis) on a laboratoryjar roller at about 100 rpm. The charge for each run was 100 to 200 g.of -325 mesh particle size rich gold bronze feed, grade B118 (nominalcontent 71.0% Cu, 28.75% Zn, 0.25% Al), and the TFE used was Halon TFE,type G-SO (a fine powder). After each run the balls were separated outand the material screened with a 325 mesh screen. The 325 mesh portionof the produced flake was taken as the product, tested for leafing andwater coverage and put in a lacquer coating. Leafing was measured by aleafing test as set forth in ASTM specification No. D-267-41.

In a first run, the jar containing about 2 kg. of balls inch to inch indiameter was charged with 100 g. of the rich gold bronze feed and 1.0 g.'IFE powder and rotated at about 90 rpm. for 6 hours. The producedflakes were smooth and brilliant but very thick. A second run, madeunder identical conditions but with the milling time increased to 16hours, produced 65 g. of 325 mesh gold bronze flakes which again showedhigh cleanliness (i.e., freedom from fines) and in addition exhibitedgood leafing properties.

In a third run, a charge of the gold bronze feed with 2% TFE powder wasmilled for 21 /2 hours and the produced -325 mesh flakes were thenpolished in a laboratory polisher for 3 hours. It was observed that thismaterial was improved in each of the three hours of polishing, althoughgold bronze flakes made by conventional methods begin to break down andto deteriorate in general quality after about 1% hours of polishing bythis laboratory polishing procedure. The polished product was found tohave a water coverage of 2100 cm. g. and leaf In a further run, with2750 g. of balls in the jar, a charge of 200 g. of the 325 mesh richgold bronze feed initially mixed with 1% TFE powder was milled forhours, with addition of TFE powder during milling the total 5%. Theproduct was polished for three hours and then had a water coverage of1350 cmF/g.

Another run, made with 200 g. of the rich gold bronze feed milled with1% TFE powder for 20 hours at 107 r.p.m., produced 325 mesh gold bronzeflakes having a water coverage of 1100 cm. /g. and leaf 42%.

A further run, in which the rich gold bronze feed was milled with TFEpowder for 22 hours, produced 40 g. of 325 mesh flakes having a watercoverage of 980 cm. /g. and leaf 31%. This product, in a paint sprayedon a steel panel, showed good resistance to discoloration when heatedfor 2 hours at 340 F.

In a further test, gold bronze 325 mesh powder produced by milling therich gold bronze feed with TFE powder in the rotating jar was polished(in conventional manner) with stearic acid for 4 /2 hours. This polishedflake had a water coverage of 2400 cnL /g. and leaf 35-40%.

In still another run, the 325 mesh gold bronze feed was mixed with +325mesh gold bronze flakes produced in a previous test and milled with TEEpowder for 5 hours. 57 g. of 325 mesh flakes were produced, having aWater coverage of 1330 cmP/g. and leaf 30%.

By way of comparison with the foregoing tests, a series of runs was madein the same equipment using 100200 g.

. of the same rich gold bronze feed but with powdered stearic acidrather than TFE powder as a grinding agent. In one such run, 200 g. ofthe metal feed Were milled with stearic acid for 7 hours; the productwas dark, and had no leaf. Another run, in which 200 g. of the metalfeed were milled with /2 stearic acid for 8 hours, produced 72 /2 g. of325 mesh flakes which again were dark, and had a Water coverage of 980cm. /g. and no leaf. In a further run, wherein a charge of the rich goldbronze feed was milled for 6 hours with stearic acid, 76 g. of 325 meshflakes were produced having a water coverage of 630 cm. g. and no leaf.

As a further comparison, a similar amount of the rich gold bronze feedwas milled in the same equipment for 5 hours with no grinding agent. Adark, gritty powder was produced, containing no flakes.

EXAMPLE II Using the equipment and following the milling procedure setforth in Example I, 200 g. of the same rich gold bronze feed were milledfor 14 /2 hours with /s% powdered stearic acid and 1% TFE powder. 87 g.of 325 mesh flakes were produced having a water coverage of 1680 cm. /g.and leaf 10%.

EXAMPLE III Again in the equipment of Example I, 200 g. of the describedrich gold bronze feed were milled for 14 hours with 1%% powderedpolychlorotrifluoroethylene (CTFE). The produced 325 mesh flakes had awater coverage of 700 cm. /g. and no leaf.

EXAMPLE IV 10 g. of the 325 mesh rich gold bronze feed were milled 11hours in the same jar mill with 1% powdered fluorinated ethylenepropylene (tetrafluoroethylcnehexafluoropropylene copolymer). Thefluorinated ethylene propylene used was Liquinite P-190. 17 g. of 325flakes were produced having a water coverage of 730 cmF/g. and leaf 22%.

EXAMPLE V g. of the same -325 mesh gold bronze feed were milled in theequipment of Example I for 8 hours with 1% powdered polyvinylidenefluoride (Kynar 401). 38 g. of -325 mesh flakes were produced having awater coverage of 670 cm. /g. and no leaf. The +325 mesh flake fractionof the product was returned to the jar with 50 g. of the -3?.5 mesh richgold bronze feed and milled for another 11 /2 hours. The 325 mesh flakeproduct of this further milling, after being polished for 2 hours, had awater coverage of 1020 crnF/g. and no leaf.

EXAMPLE VI Grinding Milling Amount of W atcr Leaf,

Agent Time (l1r.) Product (g) Coverage Percent 1 g. TFE 6% l8 1, 050 251g. O'IFE 6% 33 560 None EXAMPLE VII Again using the procedure andequipment of Example I, 100 g. of -325 mesh rich pale gold bronze feed(18.75% Zn, 0.25% Al, balance Cu) were milled with /2 g. TFE powder for6 /2 hours. 7 g. of 325 mesh flakes were screened out; the remainder ofthe feed was milled for 5 hours more, and another 20 /2 g. of -325 meshflakes were then screened out. The combined 28 g. of -325 mesh flakeshad a water coverage of 950 crn. /g. and 22% leaf.

9 EXAMPLE vm 100 g. of 200 mesh copper powder in mixture with 97 g. ofcopper flakes from a previous similar run and 2 g. of TFE powder weredry-milled for 14 hours in the jar of Example I. 41 g. of 325 meshcopper flakes were produced having a water coverage of 870 cm. /g. andleaf 19%.

EXAMPLE IX A series of dry-milling runs were made in the equipment ofExample I with feeds of aluminum bronze (i.e. copper-aluminum alloys) inpowder form. In each run, 100 g. of aluminum bronze powder feed and 1.0g. of TFE powder were charged to the jar, and the 325 mesh fraction ofthe resultant flake was taken as product. Results, for feeds of variouscopper-aluminum alloys, are summarized in the following table:

50 g. of silver crystals were dry-milled with 0.5 g. TFE powder in theequipment of Example I for 5 hours. At the end of this time, only atrace of -325 mesh flakes were screened out; the product included 1 /2g. of -l mesh +325 mesh flakes and 49 g. of +100 mesh flakes.

For purposes of comparison, 50 g. of silver crystals with 0.25 g.stearic acid powder were dry-milled in the same equipment for 1 /2 hrs.At the end of that time, 6 /2 g. of 325 mesh flake-s were screened out;the remainder (+325 mesh portion) of the material was then milled for 1hour and 20 minutes more, and from this another /2 g. of -325 meshflakes were screened out. Only 7.4 g. of +100 mesh material was presentafter this second milling period. The combined portions of '325 meshflakes had some leaf, and a water cover of 1680 cm. g.

This comparison demonstrates the absence of superfines in materialmilled with TFE.

EXAMPLE XI Aluminum powder was wet-milled with TEE powder andhydrocarbon solvent in a ball mill 3 feet in diameter and 1 foot long,to produce aluminum flake pigment. In each run, 11 /4 lb. aluminumpowder with 4% gal. hydrocarbon solvent were charged to the milltogether with the TFE powder and milled 5 hours, washed out, screenedthrough a 325 mesh screen, filtered and dried. In the first run, 153 g.(3%) of TFE powder was used, and the -325 mesh aluminum flake productwas found to have a water coverage of 12,250 cm. /g.; in the second run,255 g. (5%) of TFE powder was used, and the -325 mesh flake powder had awater cover of 11,900 cmF/g. Each product was tested for leafing bystirring in xylene, and exhibited no leaf in xylene. The xylene test isused when a strictly non-leafing product is desired, as some flakeswhich show no leaf in the leafing test referred to in Example I willshow traces of leaf in xylene.

The 325 mesh flake product of each run was sprayed out in an alkyd aminevehicle on a panel and tested for staining by placing on the panel 3drops of 5 cc. hydrochloric acid in 95 cc. of water and allowing thedrops to evaporate at room temperature, this being a convenient test oftendency to spot in automobile finishes. Both products showed verylittle HCl staining by this test, ie less HCl staining than conventionalnon-leafing aluminum flakes milled with oleic acid.

10 EXAMPLE XII In the equipment of Example I, 40 g. of aluminum powderwere wet-milled with 2 g. of TFE powder in mineral spirits, a total of260 cc. of mineral spirits being added in portions during the millingperiod of 8 hours. The jar mill was then washed out with mineral spiritsand the product filtered out. The product (tested for leafing by thetest set forth in ASTM specification No. D- 480-59T) had no leaf, andhad a water coverage of 5,600 cm. /g.; the lacquer coating gloss was 24,and :total reflectance 62.

EXAMPLE XIII In the same equipment, 40 g. of aluminum powder werewet-milled in mineral spirits for 5 hours with 1 g. TFE powder and 1 g.stearic acid powder, cc. of mineral spirits being introduced to the jarbefore grinding and 10 cc. more added during the milling operation. Theproduct flake was washed out and filtered as before. This product(tested for leafing as in Example XII) had 62% leaf, water coverage of9800 cm. /g.; lacquer coating gloss 32, total reflectance 65; andbrushed panel gloss 44, total reflectance 75.

EXAMPLE XIV In a series of runs, rich gold bronze feed (-100 meshatomized brass powder) Was dry-milled with fine TFE powder, to producegold bronze flakes, in a productionscale ball-mill operated underproduction conditions. The mill was 3 feet in diameter by 10 feet long;it contained 5,000 lb. of diameter steel balls and was rotatedcontinuously at 36 r.p.m., with continuous feed and product removal. Inoperation, air circulated through the mill removed flakes; fine flakeswere delivered to a product collector, and coarse flakes were classifiedout and returned to the mill.

At start-up, the mill was charged with 200 lb. of the atomized brasspowder and 810 g. of TFE powder, and run for one hour without aircirculation, in accordance with usual practice for beginning operationwith raw feed and :an empty mill. Air circulation, classification andproduct removal were then started, while the mill was continuouslyrotated; operation was thereafter continued for 7 /2 hours while themill was fed at a rate of 20 lb./hr. of brass powder and 81 g./hr. ofTFE powder. A total of 93 'lb. of flake product was collected. Theunfinished material was left in the mill.

Operation of the mill was resumed the following day for 11 hours, with afeed rate of about 30 lb./hr. of brass powder and 60 g./hr. of TFEpowder; 309 'lb. of product flake was collected. On a subsequent day,operation was resumed for 11 /2 hours with a feed rate of 30 lb./hr. ofbrass powder and 90 g./hr. TFE powder; 272 1b. of product flake wascollected. Thereafter on the same day) operation was again resumed for17 /2 hours with a feed rate of 30 lb./hr. brass powder and g./l1r. TFEpowder; 433 lb. of product flake was collected, and 469 lb. ofunfinished material was removed from the mill for use as initial chargein a subsequent run.

In the described operation, the hourly product collection rates indicatethat a steady production of between 20 and 30 lb./ hr. can be maintainedusing the percentages of TFE milling iagent tested. The production ofextremely fine material, i.e. dust or superfines, is very much less thanoccurs using stearic acid as a grinding agent to make a similarly fineproduct.

The mill product was then polished, the greater part being brushpolished in 50 lb. lots, 5 hours with no polishing agent, then 5 hoursmore with 50 g. stearic acid added. All the material was then blendedtogether.

Tested properties of samples of the product of the above-describedproduction-scale run are set forth in the following table:

Particle Size, Percent Apparent Water Lot Density Leaf, Coverage +150Mesh 325 Mesh (g./in. Percent (cmfl/g.)

Percent First Days Run 0.2 94. 2 12.8 Trace 840 Second Days Run 0. 2 94.2 10. 7 1, 190 Third Days Rum... Trace 94. 6 11. 3 10 1,050 FinalPolished Lot Trace 98. 4 20. 8 2, 450

A small amount of the product, polished diflerently (viz bydry-polishing, i.e. without polishing agent, for 18 hours, and thenfurther polishing for 6 hours with 1 gram of stearic acid per pound ofproduct), had an apparent density of 23.1 g./in. leaf of 70% and watercover of 2100 cm. /g., with 95.2% of the particles being of 325 meshsize.

In all the above examples of producing gold bronze flake with afluorocarbon resin grinding agent, viz. EXarnples I, II, III, IV, V, VI,VII, VIII and XIV, the bronze flake product was desirably clean, brightand free from fines.

In each of the foregoing examples, the grinding agent used was in finepowder form. However, in other runs wherein finely divided metal wasmilled with /8" pellets of fluorinated ethylene propylene and with arather coarse powder made by grinding these pellets, good flakes wereproduced, although rather slowly; in these runs, the pieces offluorinated ethylene propylene seemed to be reinforced by the particlesof brass beaten into them so that they did not readily spread out andserve the purpose of a grinding agent. In general, it is preferred touse the fluorocarbon resin grinding agents in finely divided form, assuch form is believed to facilitate the desired thin and even spreadingof the grinding agent through the feed of finely divided metal in themill.

It is to be understood that the invention is not limited to theprocedures and embodiments hereinabove specifically set forth, but maybe carried out in other ways without departure from its spirit.

I claim: a

1. A method of making flake metal powder, comprising grindingfinely-divided metal to flake form in the presence of a fluorocarbonresin as a grinding agent.

2. A method according to claim I, wherein said fluorocarbon resin is aperfluorinated linear polyolefln.

3. A method according to claim 2, wherein said fluorocarbon resin ispolytetrafluoroethylene.

4. A method according to claim 2, wherein said fluorocarbon resin isfiuorinated ethylene propylene.

5. A method according to claim 1, wherein said fluorocarbon resin ispolyvinylidene fluoride.

6. A method according to claim 1, herein said fluorocarbon resin ispolychlorotrifluoroethylene.

7. A method according to claim 1, wherein said fluorocarbon resin is infinely divided form.

3. A method according to claim I, wherein the grinding step is performedin the presence of a fluorocarbon resin and another grinding agent.

9. A method according to claim 8, wherein said fluorocarbon resin ispolytetrafluoroethylenc and said other grinding agent is stearic acid.

10. A method of making flake metal powder, comprising dry-millingfinelydivided metal to flake form in the presence of a fluorocarbonresin as a grinding agent.

11. A method of making flake metal powder comprising charging a feed offinely-divided metal and a fluorocarbon resin in finely divided form toa ball mill, said fluorocarbon resin being present in a proportion ofbetween about and about 4% of the weight of said feed, and dry-millingsaid finely-divided metal to flake form in said ball mill in thepresence of said fluorocarbon resin.

12. A method of making flake metal powder, comprising wet-millingfinely-divided metal to flake form in the presence of a liquidwet-milling vehicle and a fluorocarbon resin as a grinding agent.

13. A method of making flake metal powder, comprising charging a feed offinely-divided metal and a fluorocarbon resin in finely divided form toa ball mill together with an organic liquid wet-milling vehicle, saidfluorocarbon resin being present in a proportion of between about 1% andabout 10% of the Weight of said feed, and wetmilling said finely-dividedmetal to flake form in said ball mill.

14. A method of making flake metal powder, comprising grindingfinely-divided metal of which at least a major constituent is copper toflake form in the presence of a fluorocarbon resin as a grinding agent.

15. A method of making flake gold bronze powder, comprising grindingfinely-divided brass to flake form in the presence of a fluorocarbonresin as a grinding agent.

16. A method of making flake gold bronze powder, comprising dry-millingfinely-divided brass to flake form in the presence of a fluorocarbonresin in finely divided form as a grinding agent.

17. A method of making leafing flake gold bronze powcomprising charginga feed of finely-divided brass and polytetrafluoroethylcne infinely-divided powder form to a ball mill, said polytetrafluoroethylcnebeing present in a proportion of between about A and about 4% of theweight of said feed, and dry-milling said feed in said ball mill toreduce at least part of said finely-divided brass to flakes of -325 meshparticle size.

13. A method according to claim 17, further including the steps ofwithdrawing said 325 mesh brass flakes from said ball mill and polishingsaid withdrawn flakes.

19. A method according to ciaim 18, wherein said step of polishing theflakes inciudes the step of polishing the flakes in the presence ofstearic acid.

A method making flake aluminum pigment, comprising grindingflneiy-divided aluminum to flake form in the presence of a fluorocarbonresin as a grinding agent.

21. A method of making flake aluminum pigments, comprising wet-millingfinely-divided aluminum to flake form in the presence of a liquidwet-milling vehicle and a fluorocarbon resin, in finely-divided form, asa grinding agent.

22. A method according to claim 21, wherein said fluorocarbon resin ispolytetrafluoroethylcnc.

23. A method of making leafing flake aluminum pigments, comprisingwet-milling finely-divided aluminum to flake form in the presence ofmineral spirits and stearic acid and polytetrafluoroethylene infinely-divided form as grinding agents.

A metal flake pigment comprising a flake metal powder raving a coatingof a fluorocarbon resin, and prepared by grinding finely-divided metalto flake form in the presence of said fluorocarbon resin as a grindingagent.

25. A pigment as defined in claim 24, wherein said fluorocarbon resin ispolytctrafluoroethylene.

A pigment as defined in claim 24, wherein said fluorocarbon resin isfluorinated ethyiene propylene.

2'7. A pigment as defined in claim 24, wherein said fluorocarbon resinis polyvinylidene fluoride.

28. A pigment as defined in claim 24, wherein said fluorocarbon resin ispolychlorotrifluoroethylene.

29. A pigment as defined in claim 24, wherein said flake metal powder iscopper powder.

3d. A pigment as defined in claim 29, wherein said fluorocarbon resin ispolytetrafiuoroethyiene.

31. A pigment as defined in claim 24, wherein said flake metal powder isa copper-aluminum alloy.

32. A pigment as defined in claim 31, wherein said fluorocarbon resin ispolytetrafiuoroethylene.

33. A pigment as defined in claim 24, wherein said flake metal powder issilver powder.

34. A pigment was defined in claim 24, wherein said flake metal powderis brass powder.

35. A pigment as defined in claim 34, wherein said fluoroearbon resin ispolytetrafiuoroethylene.

36. A pigment as defined in claim 24, wherein said flake metal powder isaluminum powder.

37. A pigment as defined in claim 36, wherein said fluorocarbon resin ispolytetrofluoroethylene.

References Cited UNITED STATES PATENTS Young 26023.4 Davis 1855 Castelliet a1. 241-16 Stephens et a1. 106-290 Brown et al. 106-290 Swenson 25258Rolles et a1. 106-290 Wallen 26023.7 Owens et a1. 252--58 DONALD E.CZAJA, Primary Examiner.

R. A. WHITE, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,389,105 June 18 1968 William T. Bolger It is certified that errorappears in the above identified patent and that'said Letters Patent arehereby corrected as shown below:

Column 2, line 49, "griding" should read grinding Column 5, llne 28,"ringing" should read ranging Column 8, line 2o, "10 g." should read 100g. Column 11, line 51, "herein" should read wherein Column 12, line 44,

"A method making" should read A method of making Column 13, 11116 7,"was" should read es Signed and sealed this 4th day of November 1969.

lest:

ard M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Nesting Officer Commissioner of Patents

